A typical orthogonal frequency-division multiplexing (OFDM) signal has a relatively large peak-to-average power ratio (PAPR), since it typically consists of a number of independently modulated subcarriers. Since a high PAPR means the signal has a large dynamic range, this tends to increase the complexity of the analog/digital and digital/analog converters (ADC/DAC) required, and also tends to reduce efficiency of the radiofrequency power amplifier (PA). The large PAPR also tends to put a more stringent requirement for linearity on the power amplifier. Further, a higher input back-off factor would be needed to be applied to the signal, due to power amplifier nonlinearity, which may result in significant signal distortion.
In the case of millimeter wave (mmW), the PAPR issue may be even more severe, since the cost of a suitable ADC/DAC and the PA may be even greater. PAPR issues may be of greater concern for uplink transmissions, since the transmitters are user equipment (e.g., typically mobile devices such as cellular telephones) which have more cost, size and power constraints.
A single-carrier frequency-division multiple access (SC-FDMA) approach has been used for uplink transmissions. Although SC-FDMA has lower PAPR than an orthogonal frequency division multiple access (OFDMA) signal, a challenge remains in that SC-FDMA is unable to provide constant PAPR for all modulation levels (e.g., 16-QAM or 64-QAM), since the constellation points do not all have the same power amplitude for higher modulation levels.